1. Field of the Invention
The present invention relates to a mobile station in a mobile communication system using a closed loop transmit diversity technique in a base station and, more particularly, to an optimum weight estimating method and apparatus for selecting a desirable signal from a mobile station using a multi-path effect.
2. Description of the Related Art
Third-generation mobile communication systems are the standards for transmitting data at a higher rate than a second-generation personal communication system (PCS). A synchronous wideband code division multiple access (WCDMA) mode is standardized as a wireless access specification in both Europe and Japan. A synchronous multi-carrier CDMA (CDMA-2000) mode is standardized as a wireless access specification in North America. Mobile communication systems are configured such that a plurality of mobile stations communicate via a single base station.
Fading should be overcome to achieve fast data transmission in a mobile communication system. Fading reduces the amplitude of a receiving signal from several decibels (dB) to several tens of decibels (dB). To overcome such a fading problem, a variety of diversity techniques have been employed.
The CDMA mode employs a rake performing diversity reception using a delay spread of a channel. The rake is a multi-path diversity technique. This diversity technique has a disadvantage in that it does not operate when a delay spread is small. Time diversity using interleaving and coding schemes is employed in channels having a Doppler spread. It is difficult to use this diversity in a slow Doppler channel.
Space diversity is used in an indoor channel having a small delay spread and a pedestrian channel falling under a slow Doppler channel to overcome fading. The space diversity technique uses at least two antennas. When a signal forwarded from one antenna is reduced due to fading, the signal forwarded from the other is received.
The space diversity technique is divided into receive diversity using receiving antennas and transmit diversity using transmitting antennas. Since it is difficult to apply the receive diversity in aspect of the area of a mobile terminal (or a mobile station) and the installing cost, use of transmit diversity in a base station has been recommended.
The transmit diversity includes closed loop transmit diversity operating based on downlink channel information fed back from a mobile station and open loop transmit diversity without feedback. When using L antennas, the closed transmit diversity is more beneficial than the open transmit diversity by L times in terms of a signal to interference/noise ratio (SINR).
The performance of the closed loop transmit diversity operating based on feedback channel information is influenced by a feedback period. When the feedback period is long, a channel changes before feedback information arrives at a mobile station, thereby decreasing performance. When a large amount of information is fed back over a unit time to track a fast changing channel, uplink capacity drops, thereby decreasing performance.
The transmit diversity is divided into a maximal ratio combine (MRC) mode, an equal gain combine (EGC) mode and a selective combine (SC) mode according to a diversity combine mode. Unlike the MRC mode, the EGC mode does not consider the difference in gain between two antennas, and thus performance is degraded.
The SC mode selects a signal of an antenna having the largest gain. When a signal is received through a multi-path, determining an optimum weight for a multi-transmitting antenna becomes more complicated as the number of transmitting antennas increases. There is enough time to determine an optimum weight since calculation is performed once during each period of weight feedback, but a simpler weight determination algorithm is desired in aspect of hardware and power efficiency.
U.S. Pat. Nos. 5,634,199 and 5,471,647 relate to using transmit diversity as a feedback mode. These patents propose a feedback method and a channel measurement using a perturbation algorithm and a gain matrix, but this method is a blind method and is not usually used in a system having a pilot because a convergent speed is slow, and it is more difficult to find an accurate weight according to this method as compared with the present invention.
There has also been proposed a method of searching for an optimum channel according to an eigen method using a quantized weight vector lookup table. Channel information to be fed back should be quantized. The amount of calculation is reduced since quantization is considered in a step of obtaining a weight.
This method is based on a theory that an eigen vector corresponding to the maximum eigen value of the correlation matrix of a channel matrix is an optimum weight vector with the correlation matrix being obtained from channels received through a multi-path and an antenna path. All weight vector stored in the lookup table are applied to an estimator, and a weight vector that maximizes receiving power is set as an optimal weight vector. In this method, receiving power should be calculated with respect to all the weights, so that the amount of calculation considerably increases when there are many antennas.
A feature of an embodiment of the present invention provides an apparatus and method for minimizing power loss in a mobile station by reducing the number of calculation of weights in transmit diversity employed for improving the performance of the mobile station.
According to another feature of an embodiment of the present invention, there is provided an optimum weight estimator of a mobile station in a mobile communication system in which a base station uses closed transmit diversity technology. The optimum weight estimator includes a channel separator for separating a multi-path channel from a signal of the base station and outputting a channel matrix signal, a weight vector set generator for encoding a weight using a fixed point method and outputting a weight vector matrix signal, a weight vector determiner for outputting an optimum weight in response to the channel matrix signal and the weight vector matrix signal, and an optimum weight feedback unit for outputting a signal for feeding back the optimum weight signal to the base station.
In accordance with another aspect of an embodiment of the present invention, there is provided an optimum weight estimating method of a mobile station in a mobile communication system in which a base station uses closed transmit diversity technology. The optimum weight estimating method includes the steps of (a) separating a multi-path channel from a signal of the base station and outputting a channel matrix signal, (b) encoding a weight using a fixed point method and outputting a weight vector matrix signal, (c) outputting an optimum weight in response to the channel matrix signal and the weight vector matrix signal, and (d) outputting a signal for feeding back the optimum weight signal to the base station.